1. Field of the Invention
The present invention relates to a method for heat-treating a thermoplastic film useful for a polarizing plate, an optical compensatory film for a liquid crystal display panel, an antireflection film, a liquid crystal display device, etc., and to a thermoplastic film and a method for producing the same.
2. Description of the Related Art
For the purpose increasing the viewing angle of a liquid crystal display device, a thermoplastic film is stretched to generate an in-plane retardation and a thickness direction retardation, and used as a phase difference film for the device. Particularly in recent years, a phase difference film capable of achieving a high contrast in a liquid crystal display device has been required to apply TN display mode to televisions.
Stretched thermoplastic films have been known as the phase difference film. The thermoplastic film can be stretched by using a longitudinal stretching method for stretching in the longitudinal (length) direction, a transverse stretching method for stretching in the transverse (width) direction, a successive biaxial stretching method for stretching in the longitudinal and transverse directions successively, or a simultaneous biaxial stretching method for stretching in the longitudinal and transverse directions simultaneously. However, these methods are disadvantageous in that, for example, the resultant stretched film cannot achieve the high contrast in a liquid crystal display device, or the method is poor in productivity. A method for producing a phase difference film, using uniaxial stretching, is described in Japanese Laid-Open Patent Publication No. 2001-305342. However, this method is extremely poor in productivity because the phase difference film produced by the method has a slow axis along the longitudinal direction, and thereby cannot be attached to a polarizer or a polarizing plate in a roll-to-roll manner. A film having an NZ value of 0.90 to 1.20 and a slow axis along the transverse direction is proposed in Japanese Laid-Open Patent Publication No. 2007-108529. Further, a film having an NZ value of 0.9 to 1.1, obtained by carrying out transverse stretching and longitudinal relaxation simultaneously, is described in Japanese Laid-Open Patent Publication No. 2006-133720. However, in the case of using these films, as the screen size of the liquid crystal display device is increased, the contrast difference (contrast unevenness) between the center and edge of the screen is increased by a thermotreatment, and also light leakage is increased.